Because of the rapidly rising costs of energy, the incentives to conserve energy are increasing, both for domestic users as well as industrial users. For most domestic users in the southern regions of the United States, home air conditioning accounts for a substantial portion of the annual energy expense. Although most dwellings are insulated, a substantial amount of energy is expended by the air conditioning compressor to pump the radiant heat absorbed by the dwelling structure out of the air conditioned living space into the outside ambient air.
The air conditioning load is intensified by the thermal storage effect associated with the attic air space found in most dwellings. Air circulates very slowly in the attic air space so that its temperature rises rapidly as solar radiation is absorbed. As a result, a large amount of heat is transferred from the exposed roof structure to the air trapped within the attic air space. The body of air trapped in the attic space acts as a thermal reservoir which transfers heat through the ceiling and into the conditioned living space. Because of the large thermal mass associated with the attic air space and the roof structure, heat transfer through the ceiling and into the living space may continue long after sundown.